As an adddition to conditional blending, gradient blending might be relatively easy to implement. What I mean is that the blending gets modified by x/y coordinate along an arbitrary gradient across the image.

Use the four-slider design or definition of the key points of the gradient. (0, transition, 100, transition, 0) already there, add a dial to select the angle and an invert switch and the gradient is quite easily defined.

With a gradient like this, things like miniature effect, separating effects at the horizon or any line will be quite simple to do.

Add a gui that allows to define the gradient with the mouse/pointer and that will be even nicer.

From what I have seen people do with masking it seems that a lot of uses would be covered by this. There's a lot of horizon-bound masks around. So the pain of waiting for bitmap masks will be lessened with relatively little effort.